Several years ago, a lab in the United Kingdom found that more than 3 percent of blood samples contained toxic amounts of vitamin D. † “Some people even slurp them from the bottle. I think there is a perception that all vitamins are good for you and that more is better than less, which is absolutely not the case.”
dr. Brewer believes that some groups benefit from doses higher than the officially recommended 10mcg per day, including those over 50, for whom she prescribes 50mcg. However, very high doses are potentially dangerous because vitamin D is fat-soluble and the body does not have a satisfactory mechanism to excrete any excess.
dr. Duane Mellor, senior lecturer at Aston Medical School, advises most people to stick to the recommended 10mcg, although a maximum of 100mcg per day is generally considered safe. Some people with a diagnosed deficiency may need more, but should only be taken under medical supervision.
“There doesn’t seem to be any real benefit to taking more unless you’re deficient,” says Dr. Mellor. He says there’s no real way to know if you’re deficient except by getting a blood test; you can ask your GP if you have any symptoms, or if private examination is available in some medical clinics. Deficiency symptoms, including stomach cramps, depression and bone pain, tend to develop over time.
“Very low long-term vitamin D levels affect your bone health because you don’t absorb as much calcium from your food and your body balances that out by getting it from your bones.” Dr. Mellor says. This can lead to conditions such as rickets in children and bone pain caused by a condition called osteomalacia in adults.
But anyone taking vitamin D supplements should be sure they don’t accidentally ingest too much, as the way doses are measured and printed on the package can be confusing. The word micrograms is sometimes written with the Greek symbol μ followed by the letter g (μg). And sometimes the strength of vitamin D is expressed in international units (IU) rather than micrograms. dr. Mellor cautions not to confuse the two: 1mcg = 40 IU, so the recommended daily allowance in the UK is 400IU or 10mcg.
The maximum recommended daily limit in the UK – of diet and supplements combined – is 100mcg for adults and children aged 11 – 17; 50mcg for children aged 1 – 10 years; and 25mcg for infants. Be sure to consider foods like breakfast cereals and milk that are sometimes fortified with vitamin D.
If you choose to take supplements, vitamin D3 (cholecalciferol) is more effective than D2 (ergocalciferol) according to the NHS. And some studies have shown that taking a vitamin D supplement with the largest meal of the day improves absorption.
dr. Brewer also advises caution when taking other supplements, especially iron, vitamin A (especially for pregnant women), and selenium. “For this, there is a narrow window between a dose that is desirable and a dose that is toxic, and it can be easy to take too much.”
In other words, when it comes to vitamins, you can definitely have too much of a good thing.